This invention relates generally to methods for use in gene expression and cell fusion techniques, particularly in the production of multi-component proteins.
Recombinant DNA techniques have been used for production of heterologous proteins in transformed host cells. Generally, the produced proteins are composed of a single amino acid chain or two chains cleaved from a single polypeptide chain. More recently, multichain proteins such as antibodies have been produced by transforming a single host cell with DNA sequences encoding each of the polypeptide chains and expressing the polypeptide chains in the transformed host cell (U.S. Pat. No. 4,816,397).
The basic immunoglobulin (Ig) structural unit in vertebrate systems is composed of two identical xe2x80x9clightxe2x80x9d polypeptide chains (approximately 23 kDa), and two identical xe2x80x9cheavyxe2x80x9d chains (approximately 53 to 70 kDa). The four chains are joined by disulfide bonds in a xe2x80x9cYxe2x80x9d configuration, and the xe2x80x9ctailxe2x80x9d portions of the two heavy chains are bound by covalent disulfide linkages when the immunoglobulins are generated either by hybridomas or by B cells.
A schematic of the general antibody structure is shown in FIG. 1. The light and heavy chains are each composed of a variable region at the N-terminal end, and a constant region at the C-terminal end. In the light chain, the variable region (termed xe2x80x9cVLJLxe2x80x9d) is the product of the recombination of a VL gene to a JL gene. In the heavy chain, the variable region (VHDHJH) is the product of recombination of first a DH and a JH gene, followed by a DHJH to VH recombination. The VLJL and VHDHJH regions of the light and heavy chains, respectively, are associated at the tips of the Y to form the antibody""s antigen binding domain and together determine antigen binding specificity.
The (CH) region defines the antibody""s isotype, i.e., its class or subclass. Antibodies of different isotypes differ significantly in their effector functions, such as the ability to activate complement, bind to specific receptors (Fc receptors) present on a wide variety of cell types, cross mucosal and placental barriers, and form polymers of the basic four-chain IgG molecule.
Antibodies are categorized into xe2x80x9cclassesxe2x80x9d according to the CH type utilized in the immunoglobulin molecule (IgM, IgG, IgD, IgE, or IgA). There are at least five types of CH genes (Cxcexc, Cxcex3, Cxcex4, Cxcex5, and Cxcex1), and some species (including humans) have multiple CH subtypes (e.g., Cxcex31, Cxcex32, Cxcex33, and Cxcex34 in humans). There are a total of nine CH genes in the haploid genome of humans, eight in mouse and rat, and several fewer in many other species. In contrast, there are normally only two types of light chain constant regions (CL), kappa (xcexa) and lambda (xcex), and only one of these constant regions is present in a single light chain protein (i.e., there is only one possible light chain constant region for every VLJL produced). Each heavy chain class can be associated with either of the light chain classes (e.g., a CHxcex3 region can be present in the same antibody as either a xcexa or xcex light chain).
A process for the immortalization of B cell clones producing antibodies of a single specificity has been developed involving fusing B cells from the spleen of an immunized mouse with immortal myeloma cells. Single clones of fused cells secreting the desired antibody could then be isolated by drug selection followed by immunoassay. These cells were given the name xe2x80x9chybridomaxe2x80x9d and their antibody products termed xe2x80x9cmonoclonal antibodies.xe2x80x9d
The use of monoclonal antibodies as therapeutic agents for human disease requires the ability to produce large quantities of the desired antibody. One approach to increased production was simply to scale up the culture of hybridoma cells. Although this approach is useful, it is limited to production of that antibody originally isolated from the mouse. In the case where a hybridoma cell produces a high affinity monoclonal antibody with the desired biological activity, but has a low production rate, the gene encoding the antibody can be isolated and transferred to a different cell with a high production rate.
In some cases it is desirable to retain the specificity of the original monoclonal antibody while altering some of its other properties. For example, a problem with using murine antibodies directly for human therapy is that antibodies produced in murine systems may be recognized as xe2x80x9cforeignxe2x80x9d proteins by the human immune system, eliciting a response against the antibodies. A human anti-murine antibody (HAMA) response results in antibody neutralization and clearance and/or potentially serious side-effects associated with the anti-antibody immune response. Such murine-derived antibodies thus have limited therapeutic value.
One approach to reducing the immunogenicity of murine antibodies is to replace the constant domains of the heavy and light chains with the corresponding human constant domains, thus generating human-murine chimeric antibodies. Chimeric antibodies are generally produced by cloning the antibody variable regions and/or constant regions, combining the cloned sequences into a single construct encoding all or a portion of a functional chimeric antibody having the desired variable and constant regions, introducing the construct into a cell capable of expressing antibodies, and selecting cells that stably express the chimeric antibody. Examples of methods using recombinant DNA techniques to produce chimeric antibodies are described in PCT Publication No. WO 86/01533 (Neuberger et al.), and in U.S. Pat. No. 4,816,567 (Cabilly et al.) and U.S. Pat. No. 5,202,238 (Fell et al.).
In another approach, complementarity determining region (CDR)-grafted humanized antibodies have been constructed by transplanting the antigen binding site, rather than the entire variable domain, from a rodent antibody into a human antibody. Transplantation of the hypervariable regions of an antigen-specific mouse antibody into a human heavy chain gene has been shown to result in an antibody retaining antigen-specificity with greatly reduced immunogenicity in humans (Riechmann et al. (1988) Nature 332:323-327; Caron et al. (1992) J. Exp. Med 176:1191-1195).
Another approach in the production of human antibodies has been the generation of human B cell hybridomas. Applications of human B cell hybridoma-produced monoclonal antibodies have promising potential in the treatment of cancer, microbial infections, B cell immunodeficiencies associated with abnormally low antibody production, and other diseases and disorders of the immune system. Obstacles remain in the development of such human monoclonal antibodies. For example, many human tumor antigens may not be immunogenic in humans and thus it may be difficult to isolate anti-tumor antigen antibody-producing human B cells for hybridoma fusion.
For a given disease indication, one antibody isotype is likely to be greatly preferred over another. The preferred isotype may vary from one indication to the next. For example, to treat cancer it may be desirable that the binding of an antibody to a tumor cell result in killing of a tumor cell. In this case, an IgGi antibody, which mediates both antibody-dependent cellular cytotoxicity and complement fixation, would be the antibody of choice. Alternatively, for treating an autoimmune disease, it may be important that the antibody only block binding of a ligand to a receptor and not cause cell killing. In this case, an IgG4 or IgG2 antibody would be preferred. Thus, even in a situation where a high affinity, antigen-specific, fully human antibody has been isolated, it may be desirable to re-engineer that antibody and express the new product in a different cell.
The growing use of phage display technology also points to a need for antibody engineering and expression methodologies. Phage display technology is used for producing libraries of antibody variable domains cloned into bacteria. This allows variable domains of desired specificity to be selected and manipulated in vitro. While bacteria offer a great advantage for selecting and producing antibody fragments, they are not capable of producing full-size intact antibodies in native configuration, and it is necessary to reconstitute fragments selected in bacteria into intact antibodies and express them in eucaryotic cells.
The present invention features a method of producing a multimeric protein from a hybrid cell formed from the fusion of two or more cells, each of which cell is engineered to express one component of the multimeric protein, as well as a method for screening for successful fusion of the cells to produce a desired hybrid cell. The methods of the invention are widely applicable to the production of proteins having two or more components.
In one specific application of the method of the invention, the multimeric protein is an antibody composed of antigen-specific heavy and light chains. DNA encoding the desired heavy chain (or a fragment of the heavy chain) is introduced into a first mammalian host cell, while DNA encoding the desired light chain (or a fragment of the light chain) is introduced into a second mammalian host cell. The first transformed host cell and the second transformed host cell are then combined by cell fusion to form a third cell. Prior to fusion of the first and second cells, the transformed cells may be selected for specifically desired characteristics, e.g., high levels of expression. After fusion, the resulting hybrid cell contains and expresses both the DNA encoding the desired heavy chain and the DNA encoding the desired light chain, resulting in production of the multimeric antibody.
In one aspect the invention features the multimeric protein produced by the method of the invention. In one embodiment, the invention includes an antibody produced by the method of the invention.
In another aspect the invention features a method for screening for successful fusion of a first cell containing a first nucleotide sequence encoding a desired antibody heavy chain and a second cell containing a second nucleotide sequence encoding a desired antibody light chain, the method comprising including a nucleotide sequence encoding a first marker gene in the first cell, including a nucleotide sequence encoding a second marker gene in the second cell, fusing the first and second cells to produce a fused cell and assaying for the presence of the first and second marker genes in the fused cell.
One advantage of the method of the invention is that cells expressing a single component of the final multi-component protein can be individually selected for one or more desired characteristics, such as a high rate of production.
Another advantage is that the method generates a cell which produces an antibody at a multiplication high rate through the fusion of two kinds of cells which are each selected prior to fusion for high production of the desired heavy or light chains.
Another advantage is that the final multi-component protein is not expressed until all the cells expressing the individual components of the multi-component protein are fused into a single hybrid cell.
Other aspects, features, and advantages of the invention will become apparent from the following detailed description, and the claims.